The present invention is directed to removeable hot melt pressure sensitive adhesive compositions prepared from styrene-isoprene-styrene block copolymers, low softening point aliphatic resins and metallic salts of fatty acids.
Hot melt adhesives are 100% solid materials which do not contain or require any solvents. They are solid materials at room temperature but, on application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form and gains its cohesive strength. In this regard, hot melt adhesives differ from other types of adhesives which achieve the solid state through evaporation or removal of solvents or by polymerization.
Hot melt adhesives may be formulated to be relatively hard and free of tack or, in contrast, to be pressure sensitive, i.e., relatively soft and tacky at room temperature. At present, there is a growing acceptance of employing hot melt adhesives in affixing labels. Pressure sensitive hot melt adhesives for labeling are usually categorized as either removable or permanent. Permanent adhesives are so formulated that once removed from the release liner applied to a substrate such as glass, metal, or plastic, they tear on any attempt to remove them and are sometimes referred to as "tamperproof". The removable type, once applied to a substrate must be able to be removed from the substrate after a residence time on the substrate and at various temperatures the substrate may be subjected to. It must cleanly release leaving no residue and without tear of the label stock as in a permanent adhesive application.
Solid hot melt adhesives for permanent adhesives have been widely used for many years, however, a pressure sensitive hot melt that gives good removability has not been available. Current removable adhesives are supplied for label stock from acrylic latices and solvented solution adhesives. Both of these materials have high molecular weight polymers that reduce flow on a surface to prevent build up of adhesion. In contrast, hot melt pressure sensitive adhesives are based on materials having lower molecular weight polymers and high amounts of very low molecular weight components that make reduced flow or wetting on a surface very difficult.
It is therefore an object of the present invention to provide a hot melt pressure sensitive adhesive suitable for use in applications where good removability and resistance to adhesion build-up on aging are important criteria.